It is well known that a clean room, a clean bench and so on are widely made use of in the process of manufacturing LSI's and LCD's. For instance, in case of manufacturing a DRAM of 1-megabit from a bare wafer (silicon wafer), there is required a semiconductor manufacturing line consisting of the manufacturing steps of about 200. Also, in case of manufacturing a TFT of the 9.4 type from a bare glass plate (LCD substrate), there is required a LCD panel manufacturing line consisting of the manufacturing steps of about 80. In such a manufacturing line, it is not possible in practice to continuously convey the object good e.g. silicon wafer or LCD substrate from process to process up to the goal without leaving the object good in any standby position. Consequently, it may happen that semifinished products are held in a certain place or area for hours being exposed to the air of the working space. For instance, in the manufacturing line of the TFT-LCD, the half-finished LCD substrate which has been finished up to the formation of an electric circuit through the prior steps, is left in a standby position in a proper carrier or a storage compartment or room for several to several tens hours being exposed to the ordinary clean room air.
As such, when the silicon wafer or the LCD substrate are left in the ordinary clean room air for a long time, it often happens that some organic substances are deposited on the surface of the wafer or the LCD substrate. If such deposition of organic substances occurs, it causes the following an inconvenient state. That is, if an insulating oxide film (SiO.sub.2) is formed on the silicon wafer surface on which organic substances are deposited, the carbon component of the organic substance is captured within the silicon dioxide film, which reduces the dielectric strength of the silicon dioxide film, thus a leakage current being increased to a great extent. Furthermore, adsorption of the organic substances disturbs the close contact between the silicon wafer surface and a photoresist film applied thereto, thus, causing failure in light exposure and etching in the photolithographic process, and resulting in incorrect formation of a circuit pattern. In addition, adsorption of the organic substances results in increasing the surface resistivity of the silicon dioxide film formed on the silicon wafer, which causes the silicon wafer to be easily charged, thereby electrostatically adsorbing the fine airborne particles thereon, and readily causing the dielectric breakdown thereof. Still further, if organic impurities contained in the clean room air are irradiated by the ultra-violet rays emitted from the optical instruments, an optical CVD reaction might take place in the clean room air, so that products generated through such reaction might stick to and cloud up the surfaces of the lens and the mirror constituting an optical instrument such as a light projector, thereby reducing the optical efficiency.
Organic substances contained in the air of the working space can give a similar ill influence to the glass substrate as the LCD substrate. In case of forming an amorphous silicon film (a-Si) for use in a thin film transistor (TFT) on the LCD substrate, if organic substances contained in the air of the working space is deposited on the LCD substrate surface, it again disturbs the close contact between the a-Si film and the LCD substrate surface. As described in the above, organic substance flying about in the air of the working space is really an ill factor to be removed as complete as possible.
On one hand, it is possible to remove the organic substance deposited on the substrate surface by some cleaning methods, for instance by means of ultraviolet/ozone treatment. According to this cleaning method, however, at least several minutes have to be spent for completely cleaning a single substrate. Consequently, enormous time has to be shared for cleaning a large number of substrates, thus naturally reducing productivity. Recently, therefore, in addition to problem with regard to the Si and LCD substrate surface contamination caused by metallic impurities and fine airborne particles contained in the air of the working space, the deep concern is focused on organic impurities which still exist even in the clean air of the working space and give a heavy influence over the production yield of semiconductor devices and others. For instance, in the article entitled `Forecast Of Airborne Molecular Contamination Limits For The 0.25 Micron High Performance Logic Process` of Technology Transfer #95052812A- TR published by SEMATECH (U.S.A.) on May 31, 1995, the organic contaminant control level (allowable surface contamination value) is discussed and made into the following Table 1. In the discussion, the article writer(s) forecasts that in 1998, the allowable number of carbon atoms on the semiconductor substrate surface should be limited up to 5.times.10.sup.13 carbon atoms/cm.sup.2 before processing and 1.times.10.sup.15 carbon atoms/cm.sup.2 after processing.
TABLE 1 ______________________________________ Requirements (Year) 1995 1998 2001 2004 2007 2010 ______________________________________ Feature Size 0.35 0.25 0.18 0.13 0.1 0.07 (micrometer) Front End Processes Organics 1E14 5E13 3E13 1E13 5E12 3E12 (atoms of C/cm.sup.2) Back End Process Organics 1E15 1E15 1E15 &lt;1E15 &lt;1E15 &lt;1E15 (atoms of C/cm.sup.2) ______________________________________
Heretofore, a chemical filter such as an activated charcoal filter has been used as means for adsorbing and removing gaseous organic impurities which are still contained in the cleaned air. A predetermined container in the form of a tower which is filled with activated charcoal is well known as the most popular and simple chemical filter. It is also well known that a chemical filter is formed in various types. For instance, some chemical filters are made in the form of a felt by interweaving fibrous activated charcoal with a binder of polyester having a low melting point or nonwoven polyester fabric, and the others are formed by adhering granular activated carbon on an urethane foam or nonwoven fabric with a proper adhesive and then fabricating it in the shape of a block or cutting it in the shape of a sheet.
In case of a clean room of which the ceiling is used as a clean air outlet, the most effective filter arrangement for removing gaseous organic impurities in the air may be achieved by arranging the chemical filters on the upstream side of the particle filters installed in the ceiling portion so as to entirely cover it. However, according to the Fire Laws of Japan, activated charcoal is classified into an inflammable material group, and it strictly requires careful attention to fire. Accordingly, in view of the disaster prevention, it should be as avoided as possible to arrange the chemical filter using activated charcoal on the backside of the ceiling of the clean room.
The clean room for use in the manufacture of LSI's and LCD's supplies the clean air to the working space, the clean air being typically controlled to have a temperature of 23 to 25 .degree.C. and a relative humidity of 45.+-.10%, respectively. In general, however, activated charcoal has a very weak hydrophobic nature, so that it adsorbs not only gaseous organic impurities but also a considerable amount of moisture even if it is placed in the air conditioned circumstances. The adsorption quantity of moisture in the air by activated charcoal is rapidly increased with a small increase of the relative humidity. For instance, after holding fresh activated charcoal adsorbing none of moisture in a container filled with a dry air, if this activated charcoal is abruptly transferred to the clean room and exposed to the clean room air of which the temperature is 23 to 25.degree. C. and the relative humidity is 45.+-.10%, activated charcoal would adsorb a large quantity of moisture before reaching its saturated state. Accordingly, if the chemical filter using fresh activated charcoal is installed on the air supply side of the clean room, the humidity level of the supplied air is lowered due to the large amount of moisture adsorption by activated charcoal, even though the humidity is controlled at a preset level on the upstream side of the filter. If the humidity is lowered exceeding a preset level, it would facilitate generation of static electricity, which often interferes with the manufacture of LSI's and LCD's. In order to solve this problem, the manufacturer of chemical filters using activated charcoal is requested to carry out a very complex work prior to their shipping the filter. That is, they first survey the temperature and humidity of their client's clean room air before their shipping the filter, and then they intentionally humidify the filters so as to meet the air temperature and humidity level in the client's clean room, and further air-tightly pack the humidified filter for shipping thereof.
A chemical filter of the tower type generally has a high adsorption efficiency, but it has a high pressure loss (vent resistance) as its drawback. On one hand, a chemical filter of the felt or sheet type is acceptably ventilative and shows the adsorption efficiency in adsorption of organic impurities which is not so different from that of the tower type chemical filter. However, there is a possibility that the filter of this type may act as a source of gaseous organic impurities. More specifically, the problem exists in various adhesives used for firmly fixing activated charcoal on the base filter media (e.g. nonwoven fabric) and/or sealing materials used for fixing the base filter media to the filter frame. For instance, neoprene resin, urethane resin, epoxy resin, silicone resin, etc. are typically used as adhesives, and neoprene rubber, silicone rubber, etc. are typically used as sealants. Thus, there is a possibility that gaseous organic impurities originating from these adhesives and sealing materials are included in the air passing through the chemical filter and may give an ill influence to the manufacture of semiconductor devices. In other words, the chemical filter of the felt or sheet type may remove the organic impurities of the ppb order contained in the clean room air, but it releases gaseous organic impurities by itself and again mixes them with the clean air once filtered by itself.
In general, materials which release gaseous organic impurities as mentioned above are also included in a constituent of the particle filter for removing particulate impurities. This filter is typically arranged on the downstream side of the chemical filter for adsorbing gaseous organic impurities, so that the particle filter may undesirably act as a gaseous organic impurity source.
A JP unexamined patent publication No. 61-103518 and a JP unexamined patent publication No. 3-98611 respectively disclose a filter which can be convertibly used as a chemical filter. The former patent publication describes a filter which is developed for removing offensive odor. This filter is formed by immersing a base material such as a urethane foam in an aqueous solution containing activated charcoal powder, adhesive of the emulsion type, and solid acid, and then drying it. In case of this filter, however, gaseous organic substances may be also released not only from the synthetic rubber latex and other organic adhesive dispersoid used as the adhesive of the emulsion type, but also from the urethane foam itself. On the other hand, the above latter patent publication describes a filter which is developed for removing harmful gas and offensive odor against human health. This filter requires combination use of the adsorbent and the organic binder as a necessary condition. However, polyethylene and other organic substances, which are described as preferable examples of the organic binder in the publication, may never fail to release undesirable gaseous organic substances by themselves.
As a filter of this kind, a JP unexamined patent publication No. 63-310636 (JP Patent No. 2,579,767) and an unexamined international publication No. WO91/16971 (JP Patent No. 2,579,767) also describe a filter in which an adsorbent such as zeolite is fixed to the surface of a honeycomb matrix having perforations by using a water glass and other inorganic binder.
Accordingly, an object of the invention is to provide a filter which is safe and reliable from the standpoint of disaster prevention, and neither disturbs the controlled humidity of the clean room air nor releases gaseous organic impurities by itself, and can remove even a very small quantity of gaseous organic impurities contained in the working space, thereby protecting the substrate surface from being contaminated with such impurities. Another object of the invention is to provide a method for manufacturing the filter as mentioned above. Still another object of the invention is to provide a clean room or the like equipped with the filter as mentioned above.